Low-alloy structural steel as well as preparation method and application thereof

Abstract

The invention relates to low-alloy structural steel, which is characterized in that the low-alloy structural steel comprises the following components in percentage by weight: 0.30 to 0.37 percent of C, 0.15 to 0.35 percent of Si, 0.45 to 0.65 percent of Mn, 0.80 to 1.10 percent of Cr, 0.15 to 0.25 percent of Mo, 0.012 to 0.025 percent of V, 0.005 to 0.01 percent of Zr, 0.003 to 0.008 percent of N and the balance of Fe and inevitable impurities, and meanwhile, the content of V and the content of Zr meet the condition that (V / 51) / (Zr / 91) is equal to 3.72 to 6.65.

Description

Technical Field

[0001] This invention relates to a low-alloy structural steel. Through composition optimization, the low-alloy structural steel of this invention, while ensuring high strength, possesses excellent low-temperature toughness and outstanding weldability. This invention also provides a method for preparing the aforementioned low-alloy structural steel. Through optimization of the quenching and tempering process, the low-temperature toughness of the steel and the low-temperature toughness of the welded area can be further improved. The low-alloy structural steel of this invention is suitable for high-load components such as gears, drive shafts, and planetary carriers. Background Technology

[0002] 4140 steel is the American Society for Testing and Materials (ASTM A29 / A29M-04) standard grade, which belongs to medium carbon low alloy structural steel. The equivalent grade in China is 42CrMo.

[0003] This steel has excellent strength properties and outstanding impact toughness at room temperature. Therefore, it is widely used in various mechanical parts subjected to high impact and heavy loads. For example, this grade of steel is used in high-stress components such as hinge pins, transmission gears, hydraulic rods of some heavy engineering machinery, and drive shafts, gears and planetary carriers of automobiles.

[0004] However, 4140 steel has poor low-temperature toughness and cannot effectively cope with high loads at low temperatures, which limits its application in high-latitude and low-temperature working conditions. Furthermore, with the continuous development of large-scale equipment, the size of some parts is also getting larger and larger, making it increasingly difficult to directly cast or forge them. Many structural parts need to be manufactured separately and then welded together. However, 4140 steel has too high a carbon equivalent and poor weldability. Even with various treatments before and after welding, the ideal yield cannot be guaranteed, and the process is complex and costly.

[0005] Based on the above problems, the inventors of this invention optimized and adjusted the composition of 4140 steel, and proposed a low-alloy structural steel with excellent low-temperature toughness and outstanding weldability. Summary of the Invention

[0006] This invention provides a low-alloy structural steel that, while ensuring room temperature strength and toughness, exhibits outstanding low-temperature toughness, and the low-temperature toughness of the weld heat-affected zone after welding is also particularly outstanding. This application further provides a method for preparing the above-mentioned structural steel.

[0007] The first aspect of this invention provides a low-alloy structural steel, wherein the composition of the low-alloy structural steel, by weight percentage, is: C: 0.30-0.37%, Si: 0.15-0.35%, Mn: 0.45-0.65%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, V: 0.012-0.025%, Zr: 0.005-0.01%, N: 0.003-0.008%, with the balance being Fe and unavoidable impurities. Simultaneously, the V and Zr contents satisfy: (V / 51) / (Zr / 91) = 3.72-6.65. Among the unavoidable impurities, S ≤ 0.030% and P ≤ 0.030% are preferably controlled.

[0008] The low-alloy structural steel proposed in this invention is an improvement on the composition of traditional ASTM 4140 steel (national standard grade 42CrMo). Specifically, it includes the addition of trace amounts of V, Zr, and N. Both V and Zr have strengthening effects. Although V and Mn contribute roughly the same to Ceq (coefficient of mass), V's impact on Ceq is far less than that of C, only about 1 / 5 of C's. Furthermore, the strengthening effect of trace amounts of V and Zr on the steel is ten to dozens of times stronger than that of C and Mn. Therefore, the inventors attempted to replace the relatively high content of C and Mn with trace amounts of V and Zr, thereby reducing Ceq and improving weldability while ensuring the steel's strength. After repeated experiments, this attempt was proven effective. Based on this, the inventors obtained C, Mn, V, and Zr contents that better match the technical effect. Furthermore, to ensure the full utilization of Zr's effect, the amount of N added was also rationally determined.

[0009] The carbon equivalent Ceq=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15 of the above-mentioned low alloy structural steel is preferably not higher than 0.68%.

[0010] Of course, the selection of V and Zr as trace additives was not solely based on strength and Ceq considerations. More importantly, the inventors discovered that their addition has unexpected effects on improving the low-temperature toughness of steel and the low-temperature toughness of the weld heat-affected zone (HAZ). In particular, the inventors found that optimal low-temperature toughness of steel and HAZ low-temperature toughness can be obtained by synergistically controlling the relationship between V and Zr contents.

[0011] Below, the inventor will attempt to explain the mechanism of the technical effects produced by adding V and Zr. Some explanations may be biased due to the inventor's limited cognitive level, but the technical effects produced by the invention are objective. Therefore, even if there are inaccuracies in the mechanism explanation, it cannot negate the technical effects brought about by the addition of the above elements and the control of their content in this invention.

[0012] I. Impact on steel base materials.

[0013] Vanadium plays a role in grain refinement, primarily existing in steel as a carbide precipitate. During austenitization, VC particles pin austenite grain boundaries, hindering grain growth and significantly refining austenite grain size. Refined grains increase grain boundary area, impeding dislocation movement and crack propagation, causing cracks to deflect multiple times during propagation, consuming more energy, thus improving the low-temperature impact toughness of the base material. Furthermore, VC is a submicron or nanoscale precipitate; when uniformly dispersed, it can produce precipitation strengthening, increasing steel strength. Too low a vanadium content will not provide the corresponding strengthening and toughening effect, while too high a vanadium content can cause the precipitates to coarsen or aggregate, becoming the nucleus for crack initiation, thereby reducing low-temperature toughness and the strength of the base material.

[0014] Zirconium exhibits a more pronounced grain-refining ability, but to fully realize these benefits, the addition of an appropriate amount of nitrogen (N) is crucial. This allows Zr to form Zr(C,N) precipitates in the steel. These precipitates possess extremely high-temperature stability. During the austenitization process of steel, Zr carbonitride precipitates effectively hinder austenite grain growth, with a more significant grain-refining effect than vanadium. Finer grains mean a lower ductile-brittle transition temperature (DBTT), significantly improving the low-temperature impact resistance of the base material. Furthermore, the precipitation strengthening effect produced by finer grains is more pronounced, significantly enhancing the strength of the base material. Additionally, Zr is a strong deoxidizing and desulfurizing element, combining with O and S in steel to form inclusions such as ZrO2 and ZrS. This reduces the segregation of low-melting-point impurities (such as S and P) at grain boundaries, preventing grain boundary embrittlement and purifying the grain boundaries. Improved grain boundary cleanliness makes crack propagation along grain boundaries more difficult, further improving the low-temperature toughness of the base material. If the zirconium content is too low, the aforementioned strengthening and toughening effects will not be fully achieved. However, if the zirconium content is too high, the size, quantity, and distribution of the precipitated phases will be difficult to control. Coarse and clustered carbonitrides are prone to becoming the initiation point of cracks, reducing the strength and low-temperature toughness of the base material.

[0015] II. Impact on HAZ.

[0016] The toughness degradation of the welded hardened zone (HAZ) mainly stems from grain coarsening and the formation of brittle structures in the overheated zone. During welding, the temperature in the HAZ overheated zone can reach 1100~1300℃, causing austenite grains to coarsen rapidly and form coarse-grained regions. However, the high temperature in the weld overheated zone does not dissolve Zr (C, N), and its precipitated phases can pin the austenite grain boundaries throughout the process, completely avoiding abnormal grain coarsening in the overheated zone and fundamentally solving the problem of coarse-grained embrittlement in the HAZ. The stability of the Zr precipitated phases is not affected by fluctuations in welding heat input, and the above effects can still be stably exerted in welding with high heat input. Zr can reduce the cooling rate sensitivity of the HAZ and inhibit the formation of coarse structures. Zr (C, N) can refine the carbides in the HAZ, making them dispersed and avoiding embrittlement caused by continuous precipitation of carbides at grain boundaries.

[0017] Vanadium carbides undergo a dissolution-reprecipitation process in the hard annealing zone (HAZ). This reprecipitation typically forms coarse volatile organic compounds (VCs). Therefore, the presence of VCs is conventionally considered detrimental to HAZ toughness. However, the fine Zr (C, N) particles of this invention do not dissolve in the HAZ, providing nucleation sites for the reprecipitated VCs. This ensures that the reprecipitated VCs also pin grain boundaries in a fine and dispersed manner, inhibiting abnormal austenite grain growth, mitigating coarse grain embrittlement, and improving the low-temperature toughness of the HAZ. Furthermore, vanadium can delay the precipitation and coarsening of carbides during tempering, enhancing the tempering stability of the HAZ and maintaining a high level of low-temperature toughness.

[0018] Insufficient addition of either V or Zr will result in the aforementioned effects being insignificant, while excessive addition will easily lead to the formation of agglomerated, coarse carbides / carbonitrides. These carbides / carbonitrides are highly likely to develop into crack initiation sites, resulting in the deterioration of the low-temperature toughness of the HAZ.

[0019] It is easy to see that the synergistic effect of V and Zr composite addition in improving the strength and low-temperature toughness of the base material and the low-temperature toughness of HAZ lies in the fact that fine VC plays a role in the low-temperature range such as tempering and cooling, improving the strength of the base material by precipitation strengthening and inhibiting temper embrittlement, which can reduce the ductile-brittle transition temperature and improve the low-temperature toughness of the base material. Zr (C, N) strengthens the matrix, reduces the ductile-brittle transition temperature and purifies the grain boundaries in the high-temperature range such as hot deformation and normalizing, thereby improving strength and toughness. In particular, during the welding process, Zr (C, N) in HAZ does not dissolve but remains in a fine state. Although VC dissolves, during the post-weld cooling process, Zr (C, N) pins the grain boundaries at high temperature and induces heterogeneous nucleation of VC, avoiding VC agglomeration and coarsening, forming fine VC, which further improves the low-temperature toughness of HAZ. This makes up for the traditional technical understanding that VC cannot play a role in the heat-affected temperature region of welding and will aggravate the low-temperature toughness defect. During the hot deformation process of steel, similar to the aforementioned welding process, VC is prone to dissolution to varying degrees. The presence of Zr (C, N) can provide sites for heterogeneous nucleation of VC, prevent the VC precipitates from growing in size, ensure that VC is dispersed and finely distributed in the matrix, and improve the strength and low-temperature toughness of the base material.

[0020] As described above, in order to fully utilize the effects of V and Zr, the inventors of this invention discovered through continuous experimentation that the contents of V and Zr must be controlled within a reasonable range, namely 0.012-0.025% and Zr: 0.005-0.01%. Furthermore, to ensure the best synergistic effect of V and Zr, it is also necessary to control the content relationship between the two. Through repeated experimental exploration, the inventors determined that when the V content and Zr content satisfy (V / 51) / (Zr / 91) = 3.72~6.65, the optimal balance between strength and low-temperature toughness can be achieved.

[0021] As a further improvement, the aforementioned low-alloy structural steel is in a quenched and tempered state, resulting in a tempered sorbite microstructure. Through quenching and tempering, i.e., quenching followed by high-temperature tempering, strength and toughness can be further improved. Especially with the addition of Zr and V, during high-temperature tempering, V precipitates diffusely in the form of VC, resulting in precipitation strengthening, further enhancing strength without reducing toughness. Zr (C, N) particles are stable at the quenching and tempering temperature, pinning grain boundaries and inhibiting grain growth during tempering, further refining the microstructure and fully releasing the steel's strength and low-temperature toughness potential.

[0022] As a non-limiting description, the quenching + high-temperature tempering process can be as follows: first, hold at 850-880℃ for 1-3 hours and then quench in oil, followed by holding at 580-640℃ for 1-3 hours and then furnace cooling, air cooling, or water cooling to room temperature.

[0023] As a description of the aforementioned low-alloy structural steel's properties, its impact toughness at -40℃ after quenching and tempering is not less than 150 J / cm. 2 The tensile strength reaches over 1100 MPa, the yield strength over 930 MPa, and the elongation after fracture over 15%. After welding and tempering, the impact toughness of the weld heat-affected zone at -40℃ is not less than 110 J / cm². 2 .

[0024] As a further improvement of the present invention, the tempering treatment of the tempered state is obtained through a series of steps including graded quenching and double tempering. The graded quenching + double tempering process is a refined heat treatment scheme based on conventional tempering, and its core purpose is to further optimize the uniformity of the microstructure, reduce residual stress, and improve the strength-toughness ratio and dimensional stability.

[0025] Staged quenching can eliminate the internal and external temperature difference during the quenching process, resulting in a more uniform microstructure transformation and a smoother stress distribution during subsequent cooling. It also reduces the undercooling of the martensitic transformation, promoting the formation of fine, uniform low-carbon martensite. The uniform fine martensite structure provides a good foundation for subsequent tempering and avoids the problem of uneven performance after tempering.

[0026] The first tempering in double tempering eliminates most of the stress, while the second tempering completely eliminates residual stress, ensuring long-term dimensional stability of the workpiece. During the first tempering, martensite begins to decompose, precipitating fine cementite, but some carbides remain in a metastable state. The second tempering promotes further spheroidization and dispersion of carbides, preventing carbide segregation at grain boundaries and promoting full precipitation and uniform distribution of carbides. After double tempering, a tempered sorbite microstructure is obtained, optimizing the strength-toughness balance and improving low-temperature toughness.

[0027] For the purpose of non-limiting description, the staged quenching specifically involves first holding at 860-890℃ to complete austenitization, then cooling to 300-350℃ isothermally via a salt bath or quenching oil. The isothermal time is typically controlled at 1.0-2.0 hours to prevent bainite transformation, followed by oil cooling to room temperature. The salt bath medium can consist of 35-55% potassium nitrate and 45-65% sodium nitrite by mass.

[0028] As a non-limiting description, the double tempering step specifically involves first holding the temperature at 580-640℃ for 1.5-3.5 hours and then oil cooling to room temperature, followed by holding the temperature at 580-640℃ for 1.5-3.5 hours and then water cooling to room temperature.

[0029] As another aspect of the present invention, the present invention also provides a method for preparing the aforementioned low alloy structural steel, specifically including smelting, hot deformation, and quenching and tempering treatment.

[0030] Of course, for steel that needs to be welded, it also includes a welding step, which is performed after heat deformation and before tempering.

[0031] The tempering treatment can be either conventional quenching + high-temperature tempering as described above, or the aforementioned graded quenching + double tempering process, thereby further optimizing the uniformity of the microstructure, reducing residual stress, improving the strength-toughness match and dimensional stability.

[0032] The aforementioned low-alloy structural steel or the structural steel prepared by the aforementioned method has excellent strength and toughness, especially excellent low-temperature toughness. In the case of welding process, its HAZ low-temperature toughness is good, and it is widely used in planetary carriers of planetary gears, automotive drive shafts, gearbox gears, transmission gears, etc.

[0033] As described in detail above, the innovation of this invention lies in two aspects. First, the combined addition of V, Zr, and N, and controlling the content relationship of V and Zr within a certain range, can achieve the desired technical effects. Second, the use of a staged quenching + double tempering heat treatment process can further optimize the matching of the material's strength and low-temperature toughness. Detailed Implementation

[0034] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following description is based on specific experiments.

[0035] Experiment 1 First, steel with the compositions shown in Table 1 was smelted, with P and S controlled at 0.02% ± 0.002%. This steel was then cast into ingots (cross-sectional dimensions: 200mm × 200mm). The ingots were then hot-rolled. Before hot rolling, the steel was heated to 1150℃ and held for 2.5 hours. The initial rolling temperature was above 980℃, and the final rolling temperature was above 780℃. After final rolling, the steel was cooled to 500℃ at an average cooling rate of 5℃ / s, resulting in a 75mm thick sheet. The sheet was then subjected to a quenching and tempering treatment. The specific process parameters were: holding the sheet at 870℃ for 2 hours, followed by oil quenching, and then holding at 620℃ for 2 hours before furnace cooling to room temperature. Subsequently, the room temperature strength and elongation of the sheet were tested according to GB / T 228.1-2021, and the impact toughness (V-notch) at -40℃ was tested according to GB / T 229-2020. The results are recorded in Table 2. In Table 1, Formula 1 represents (V / 51) / (Zr / 91).

[0036] Table 1. Composition of various low-alloy structural steels, wt.%, balance being Fe.

[0037]

[0038] Table 2. Performance test results of various low-alloy structural steels.

[0039]

[0040] The elemental content and relationships of the low-alloy structural steels tested (numbers 1-6) all meet the requirements of this invention. The resulting steels exhibit room temperature tensile strength exceeding 1100 MPa, yield strength exceeding 930 MPa, elongation after fracture exceeding 15%, and impact toughness at -40℃ exceeding 150 J / cm². 2 above.

[0041] Although the elemental contents of the low-alloy structural steels tested in experiments 7-8 were all within the scope of the invention, the relationship between the contents of V and Zr did not meet the requirements of the invention. It is speculated that this is because the content of V is high and the content of Zr is low, resulting in insufficient number of Zr (C, N) precipitated particles to fix and refine the large number of VC particles dissolved and reprecipitated during hot rolling and quenching. This leads to larger VC particles, obvious agglomeration tendency, and poor dispersion, ultimately causing the deterioration of the steel's strength, plasticity, and low-temperature toughness.

[0042] The low-alloy structural steel in test number 9 had too low a nitrogen content, resulting in an insufficient number of Zr(C,N) precipitated particles. This reduced the refining and dispersion effect of Zr(C,N) on the reprecipitated VC, leading to a decrease in various mechanical properties.

[0043] The content relationship of V and Zr in test number 10 meets the requirements of the present invention, but the content of V and Zr is too high, and the amount of VC and Zr (C, N) precipitation is too large. These precipitates will form agglomerates and the coarsening trend is more obvious, which can easily develop into crack sources, resulting in a significant decrease in the strength, plasticity and low temperature toughness of the steel.

[0044] Experiment 2 The steel with the composition shown in Table 3 was smelted, with P and S controlled at 0.02% ± 0.002%. This steel was then cast into ingots (cross-sectional dimensions: 200mm × 200mm), and hot-rolled. Before hot rolling, the ingots were heated to 1150℃ and held for 2.5 hours. The initial rolling temperature was above 980℃, and the final rolling temperature was above 780℃. After final rolling, the ingots were cooled to 500℃ at an average cooling rate of 5℃ / s, resulting in multiple plates with a thickness of 75mm. Some plates were heat-treated according to the parameters shown in Table 4, while others were not. Subsequently, the room temperature strength and elongation of the plates were tested according to GB / T 228.1-2021, and the impact toughness (V-notch) at -40℃ was tested according to GB / T 229-2020. The results are recorded in Table 5. In Table 3, Formula 1 represents (V / 51) / (Zr / 91).

[0045] Table 3. Composition of low alloy structural steel, wt.%, balance Fe.

[0046]

[0047] Table 4. Heat treatment process parameters for various low-alloy structural steels.

[0048]

[0049] The above-mentioned nitrate bath consists of 35-55% by mass of potassium nitrate and 45-65% by mass of sodium nitrite.

[0050] Table 5. Performance test results of various low-alloy structural steels.

[0051]

[0052] By performing performance tests on H-steel after applying different heat treatments (or without heat treatment), it is easy to see that the mechanical properties of the cast steel samples are relatively low. The tensile strength, yield strength, elongation after fracture, and low-temperature impact toughness at -40℃ cannot meet the requirements of this application.

[0053] After tempering, all properties of the steel were improved. Compared with the traditional quenching + high-temperature tempering process, the staged quenching + double tempering process proposed in this invention can further improve the -40℃ low-temperature impact toughness of the steel, even with a slight decrease in strength, so that it can reach 180J / cm². 2 As the tempering temperature increases or the tempering time increases, the low-temperature toughness of the steel further increases, while the strength decreases slightly, but all of these are within the scope of the present invention.

[0054] The above experiments confirm that quenching and tempering can increase strength and toughness. The quenching and tempering process provided by this invention can further achieve a balance between strength and toughness in steel, and obtain higher toughness while ensuring that the strength meets the requirements.

[0055] Experiment 3 Multiple plates obtained in Experiment 2 without heat treatment were subjected to EGW (gas-electric vertical welding) with a line energy of 70 kJ / cm. After machining V-grooves on the edges of the steel plates, the plates were butt-welded vertically in a single pass. After welding, heat treatment was performed according to the heat treatment parameters in Table 6, or no heat treatment was performed. Then, specimens for testing the low-temperature toughness of the HAZ were cut in the HAZ area near the weld joint at a position 20 mm away from the surface of each thickness of steel plate. The impact toughness (V-notch) of the plate HAZ specimens at -40℃ was tested according to GB / T 229-2020, and the results were recorded in Table 7.

[0056] Table 6. Heat treatment process parameters for various low-alloy structural steels.

[0057]

[0058] Table 7. Performance test results of various low-alloy structural steels.

[0059]

[0060] Low-alloy structural steel exhibits poor low-temperature toughness (-40℃) in the hardened zone (HAZ) after welding without tempering, failing to meet the requirements for operation in extremely cold conditions. After conventional quenching followed by high-temperature tempering, the low-temperature toughness (-40℃) of the HAZ is significantly improved, reaching 110 J / cm². 2 In addition, after the tempering treatment of this invention, the low-temperature toughness of HAZ at -40℃ can reach 135J / cm. 2 This allows the steel to retain excellent low-temperature toughness after welding.

[0061] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0062] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A low alloy structural steel, characterized in that, The low-alloy structural steel has the following components in percentage by weight: C: 0.30-0.37%, Si: 0.15-0.35%, Mn: 0.45-0.65%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, V: 0.012-0.025%, Zr: 0.005-0.01%, N: 0.003-0.008%, and the balance of Fe and inevitable impurities, and the V content and the Zr content satisfy the formula (V / 51) / (Zr / 91)=3.72-6.

65.

2. The low alloy structural steel of claim 1, wherein, The low-alloy structural steel is in a quenched and tempered state.

3. The low alloy structural steel according to any one of claims 1-2, characterized in that, The low alloy structural steel has a -40℃ impact toughness of not less than 150 J / cm 2 .

4. The low alloy structural steel according to any one of claims 1-3, characterized in that, The quenched and tempered state is obtained through a step of graded quenching and a step of double tempering.

5. The low alloy structural steel according to any one of claims 1-4, characterized in that, The low-alloy structural steel after welding has an impact toughness of not less than 110 J / cm at -40 DEG C in the welding heat affected zone 2 .

6. A method of producing a low alloy structural steel according to any one of claims 1 to 5, characterized in that, The preparation method comprises the steps of melting, hot deformation, and quenching and tempering.

7. The production method according to claim 6, wherein The quenching and tempering comprises a step of graded quenching, specifically, austenitizing at 860-890℃, then cooling to 300-350℃ in a nitrate bath or quenching oil, and then oil cooling to room temperature.

8. The preparation method according to claim 6, characterized in that, The quenching and tempering comprises a step of double tempering, specifically, tempering at 550-580℃ for 1.5-3.5h, then oil cooling to room temperature, and then tempering at 550-580℃ for 1.5-3.5h, and then water cooling to room temperature.

9. The method of any one of claims 6-8, wherein, The method further comprises a welding step, which is performed after the hot deformation and before the quenching and tempering.

10. The low-alloy structural steel of claims 1-5 or the low-alloy structural steel prepared by the method of claims 6-9 is applied to a planet carrier of a planetary gear, an automobile transmission shaft, a gear of a gearbox, and a transmission gear.

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